Glutathione status and reactive oxygen generation in tissues of young and old exercised rats

The effects of exercise on the generation of active oxygen species and radical-scavenging capacity were studied in physically active and sedentary young and old rats. Exercise increased the hydroxyl radical content in all tissues of physically active young rats, except in the plasma. In old rats, the basal level of the radical increased significantly in plasma, heart, and skeletal muscles, but decreased in liver; and physical activity decreased it to that of young rats in most cases. With exercise, the content of reduced glutathione increased in plasma, heart, and skeletal muscles of young rats, whereas that of oxidized glutathione markedly decreased in liver and increased in brain and white gastrocnemius muscle. The total glutathione levels in these tissues changed in a similar way, indicating that glutathione was released from the pool in the liver. In rats allowed to run voluntarily for 5 weeks, the effects were more pronounced than in the sedentary rats. The ratio of reduced to total glutathione, which indicates the capacity to reduce glutathione, increased in plasma, heart, and soleus muscle of sedentary young rats after exercise, and increased further in those undergoing physical activity. In old rats, the reduced glutathione level increased in plasma, heart, liver, and brain, even though the total decreased. These results suggest that physical activity enhances the endogenous ability to defend against oxidative stress. In old rats, even though glutathione synthesis is decreased, the regenerating capacity seems to be increased in order to compensate for the increased oxidative stress.     

Muscle oxygen uptake in humans at onset of and during intense exercise

This review presents data on human muscle oxygen consumption in the initial phase of exercise as well as on muscle maximal oxygen uptake. It also discusses mechanistic limiting factors related to oxygen utilization at the onset of exercise and of maximal aerobic power of skeletal muscle. Direct measurements of oxygen utilization of a well-defined muscle show that contracting muscles utilize oxygen within a few seconds of exercise onset and that it takes some 45 s before oxygen extraction is maximal. The delayed oxygen utilization in the initial phase of intense exercise does not appear to be caused by insufficient oxygen availability. But it may rather be the result of a non-optimal distribution of blood flow in the exercising muscles and a limitation in the rate of oxygen extraction by the contracting muscle cells. The latter limitation does not appear to be caused by an insufficient activation of the enzyme pyruvate dehydrogenase. The maximal oxygen uptake of skeletal muscle is around 300-400 mL min-1 kg-1. This uptake rate corresponds to a TCA cycle rate of 4-5 mmol min-1 kg-1, which is of the same magnitude as the activity of oxyglutarate dehydrogenase and pyruvate dehydrogenase, suggesting that these enzymes may be rate limiting for oxygen uptake when an isolated muscle is exercising.     

Increased physical activity decreases hepatic free fatty acid uptake: a study in human monozygotic twins

Exercise is considered to be beneficial for free fatty acid (FFA) metabolism, although reports of the effects of increased physical activity on FFA uptake and oxidation in different tissues in vivo in humans have been inconsistent. To investigate the heredity-independent effects of physical activity and fitness on FFA uptake in skeletal muscle, the myocardium, and liver we used positron emission tomography (PET) in nine healthy young male monozygotic twin pairs discordant for physical activity and fitness. The cotwins with higher physical activity constituting the more active group had a similar body mass index but less body fat and 18 ± 10% higher ( P < 0.001) compared to the less active brothers with lower physical activity. Low-intensity knee-extension exercise increased skeletal muscle FFA and oxygen uptake six to 10 times compared to resting values but no differences were observed between the groups at rest or during exercise. At rest the more active group had lower hepatic FFA uptake compared to the less active group (5.5 ± 4.3 versus 9.0 ± 6.1 μmol (100 ml)⁻¹ min⁻¹, P = 0.04). Hepatic FFA uptake associated significantly with body fat percentage ( P = 0.05). Myocardial FFA uptake was similar between the groups. In conclusion, in the absence of the confounding effects of genetic factors, moderately increased physical activity and aerobic fitness decrease body adiposity even in normal-weighted healthy young adult men. Further, increased physical activity together with decreased intra-abdominal adiposity seems to decrease hepatic FFA uptake but has no effects on skeletal muscle or myocardial FFA uptake.     

Dissociation of the effects of training on oxidative metabolism,glucose utilisation and GLUT4 levels in skeletal muscle of streptozotocin-diabetic rats

The effects of long-term, moderate physical exercise on in vivo glucose uptake, levels of two glucose transporter proteins (GLUT1 and GLUT4) and activities of various key enzymes of energy metabolism were measured in skeletal muscle from streptozotocin-diabetic rats. Diabetes (12–16 weeks) reduced the in vivo glucose uptake (glucose metabolic index, GMI) in muscle containing mainly type I fibres by 55% but had no effect in muscles containing mainly type IIa and IIb fibres. GMI was increased in the diabetic white skeletal muscle (mainly type IIb fibres) by more than 120%. In contrast to the complex changes in GMI, GLUT4 levels were reduced in all types of skeletal muscle from diabetic rats with no change in GLUT1 levels. Exercise training had no effects on GMI or the glucose transporter levels. Streptozotocin induced diabetes significantly reduced the oxidative capacity of skeletal muscle assayed as the activities of citrate synthase, succinate dehydrogenase and cytochrome c oxidase. Training increased the activities of oxidative enzymes, with this increase being more prominent in the diabetic animals. The present data indicate that long-term streptozotocin-induced diabetes decreases oxidative metabolic capacity and GLUT4 protein levels in skeletal muscle, but that the changes of glucose transport largely depend on the fibre type composition. Moderate training fully reverses the effect of insulinopenia and hyperglycaemia on muscle oxidative metabolism. In contrast to the previous suggestions, the expression of GLUT4 is not correlated with the capacity of oxidative metabolism in skeletal muscle of streptozotocin-diabetic rats.     

Surgical manipulation,but not moderate exercise,is associated with increased cytokine mRNA expression in the rat soleus muscle

Interleukin (IL)-6 production in contracting skeletal muscle and IL-6 concentration in plasma are increased after prolonged and strenuous exercise. However, as tissue stress or damage are unspecific triggers of increased cytokine levels, we examined whether moderate muscle activity is an independent stimulus for cytokine expression, and to which extent invasive procedures might affect the results. Soleus muscles were isolated from sedentary rats or from rats that had been running on a treadmill at moderate intensity (70% of maximal oxygen uptake) for 1 h. In another group the soleus muscle was prepared in situ and stimulated intermittently at 5 Hz for 1 h, so that maximal developed force declined by 30%. In situ prepared soleus muscles not subjected to electrical stimulation were used as controls. Messenger RNA (mRNA) expression of 11 cytokines was analysed in the soleus muscles using multiprobe RNAse protection assay, and IL-6 plasma concentration was measured by enzyme-linked immunosorbent assay. Treadmill exercise did not affect the mRNA expression of any of the measured cytokines in the soleus muscle. Irrespective of electrical stimulation, mRNA expression of IL-6 and IL-1beta were significantly increased in the surgically manipulated soleus muscles. Interleukin-6 plasma concentration was not affected by treadmill running or electrical stimulation. Conclusion, gentle surgical manipulation is a strong stimulus for IL-6 and IL-1beta mRNA synthesis in skeletal muscle, whereas exercise or electrical muscle stimulation at moderate intensity does not independently affect cytokine mRNA levels in the contracting soleus.     

Hyperbaric oxygen modulates antioxidant enzyme activity in rat skeletal muscles

In skeletal muscle the activity of the enzymatic antioxidants superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) is regulated in response to generation of reactive oxygen species (ROS). Increased activity of these enzymes is observed after repeated bouts of aerobic exercise, a potent stimulus for intracellular ROS production. Hyperbaric oxygen (HBO) inhalation also stimulates intracellular ROS production although the effects of HBO on skeletal muscle SOD, GPx and CAT activity have not been studied. We tested the hypothesis that SOD, GPx and CAT activity is modulated in skeletal muscles in response to acute and repeated HBO administration. In adult male rats acute HBO inhalation (60 min at 3 atmospheres absolute) reduced catalase activity by approximately 51% in slow-twitch soleus muscles. Additionally, repeated HBO inhalation (twice daily for 28 days) increased Mn²⁺-superoxide dismutase activity by approximately 241% in fast-twitch extensor digitorum longus muscles. We conclude that both acute and repeated HBO inhalation can alter enzymatic antioxidant activity in skeletal muscles. Electronic Publication     

Exercise training-induced improvements in insulin action

Individuals with insulin resistance are characterized by impaired insulin action on whole-body glucose uptake, in part due to impaired insulin-stimulated glucose uptake into skeletal muscle. A single bout of exercise increases skeletal muscle glucose uptake via an insulin-independent mechanism that bypasses the typical insulin signalling defects associated with these conditions. However, this 'insulin sensitizing' effect is short-lived and disappears after approximately 48 h. In contrast, repeated physical activity (i.e. exercise training) results in a persistent increase in insulin action in skeletal muscle from obese and insulin-resistant individuals. The molecular mechanism(s) for the enhanced glucose uptake with exercise training have been attributed to the increased expression and/or activity of key signalling proteins involved in the regulation of glucose uptake and metabolism in skeletal muscle. Evidence now suggests that the improvements in insulin sensitivity associated with exercise training are also related to changes in the expression and/or activity of proteins involved in insulin signal transduction in skeletal muscle such as the AMP-activated protein kinase (AMPK) and the protein kinase B (Akt) substrate AS160. In addition, increased lipid oxidation and/or turnover is likely to be another mechanism by which exercise improves insulin sensitivity: exercise training results in an increase in the oxidative capacity of skeletal muscle by up-regulating lipid oxidation and the expression of proteins involved in mitochondrial biogenesis. Determination of the underlying biological mechanisms that result from exercise training is essential in order to define the precise variations in physical activity that result in the most desired effects on targeted risk factors, and to aid in the development of such interventions.     

Surgical manipulation,but not moderate exercise,is associated with increased cytokine mRNA expression in the rat soleus muscle

Interleukin (IL)‐6 production in contracting skeletal muscle and IL‐6 concentration in plasma are increased after prolonged and strenuous exercise. However, as tissue stress or damage are unspecific triggers of increased cytokine levels, we examined whether moderate muscle activity is an independent stimulus for cytokine expression, and to which extent invasive procedures might affectthe results. Soleus muscles were isolated from sedentary rats or from rats that had been running on a treadmill at moderate intensity (70% of maximal oxygen uptake) for 1 h. In another group the soleus muscle was prepared in situ and stimulated intermittently at 5 Hz for 1 h, so that maximal developed force declined by 30%. In situ prepared soleus muscles not subjected to electrical stimulation were used as controls. Messenger RNA (mRNA) expression of 11 cytokines was analysed in the soleus muscles using multiprobe RNAse protection assay, and IL‐6 plasma concentration was measured by enzyme‐linked immunosorbent assay. Treadmill exercise did not affect the mRNA expression of any of the measured cytokines in the soleus muscle. Irrespective of electrical stimulation, mRNA expression of IL‐6 and IL‐1β were significantly increased in the surgically manipulated soleus muscles. Interleukin‐6 plasma concentration was not affected by treadmill running or electrical stimulation. Conclusion, gentle surgical manipulation is a strong stimulus for IL‐6 and IL‐1β mRNA synthesis in skeletal muscle, whereas exercise or electrical muscle stimulation at moderate intensity does not independently affect cytokine mRNA levels in the contracting soleus.     

Overexpression of peroxisome proliferator-activated receptor gamma co-activator-1alpha leads to muscle atrophy with depletion of ATP

Peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1alpha) is a key nuclear receptor co-activator for mitochondrial biogenesis. Here we report that overexpression of PGC-1alpha in skeletal muscles increased mitochondrial number and caused atrophy of skeletal muscle, especially type 2B fiber-rich muscles (gastrocnemius, quadriceps, and plantaris). Muscle atrophy became evident at 25 weeks of age, and a portion of the muscle was replaced by adipocytes. Mice showed increased energy expenditure and reduced body weight; thyroid hormone levels were normal. Mitochondria exhibited normal respiratory chain activity per mitochondrion; however, mitochondrial respiration was not inhibited by an ATP synthase inhibitor, oligomycin, clearly indicating that oxidative phosphorylation was uncoupled. Accordingly, ATP content in gastrocnemius was markedly reduced. A similar phenotype is observed in Luft's disease, a mitochondrial disorder that involves increased uncoupling of respiration and muscle atrophy. Our results indicate that overexpression of PGC-1alpha in skeletal muscle increases not only mitochondrial biogenesis but also uncoupling of respiration, resulting in muscle atrophy.     

Low intensity training, inactivity and resumed training in sedentary men

The effects of a low intensity training regimen, consisting of two 7-week periods with an interspersed 8-week inactivity period were investigated in 16 sedentary men. A follow-up was made on 7 subjects after 38 additional weeks' training. Systemic as well as local effects were studied using exercise tests and leg muscle biopsies. The two 7-week training periods both resulted in a 6% increase in Vo2 max and a lowered heart rate during submaximal work. No persisting training effects were detected by exercise tests after inactivity. In skeletal muscle, however, striking differences in enzyme activity pattern and ultrastructure were observed between the two periods, indicating that some training effect of importance for muscle metabolic adaptation might have persisted during inactivity. It is suggested that such an effect might be associated with the local oxygen supply. During the 38-week training period there was a large increase in muscle metabolic capacity, but no change in maximal oxygen uptake. This separation of systemic and local training effects indicates a lack of a direct causal relationship between muscle metabolic potential and max imal oxygen uptake. It is suggested that the elevated muscle oxidative capacity is of importance for an increased endurance capacity.     

Disparity in regional and systemic circulatory capacities: do they affect the regulation of the circulation?

In this review we integrate ideas about regional and systemic circulatory capacities and the balance between skeletal muscle blood flow and cardiac output during heavy exercise in humans. In the first part of the review we discuss issues related to the pumping capacity of the heart and the vasodilator capacity of skeletal muscle. The issue is that skeletal muscle has a vast capacity to vasodilate during exercise [∼300 mL (100 g)⁻¹ min⁻¹], but the pumping capacity of the human heart is limited to 20–25 L min⁻¹ in untrained subjects and ∼35 L min⁻¹ in elite endurance athletes. This means that when more than 7–10 kg of muscle is active during heavy exercise, perfusion of the contracting muscles must be limited or mean arterial pressure will fall. In the second part of the review we emphasize that there is an interplay between sympathetic vasoconstriction and metabolic vasodilation that limits blood flow to contracting muscles to maintain mean arterial pressure. Vasoconstriction in larger vessels continues while constriction in smaller vessels is blunted permitting total muscle blood flow to be limited but distributed more optimally. This interplay between sympathetic constriction and metabolic dilation during heavy whole-body exercise is likely responsible for the very high levels of oxygen extraction seen in contracting skeletal muscle. It also explains why infusing vasodilators in the contracting muscles does not increase oxygen uptake in the muscle. Finally, when ∼80% of cardiac output is directed towards contracting skeletal muscle modest vasoconstriction in the active muscles can evoke marked changes in arterial pressure.     

Skeletal muscle damage: a study of isotope uptake,enzyme efflux and pain after stepping

Summary We have studied the occurrence of skeletal muscle uptake of ^99mtechnetium pyrophosphate (Tc-PYP), creatine kinase (CK) release and muscle pain in normal subjects after exercise. Five subjects stepped on and off a high bench in such a way that one leg stepped up and the other down. Pain only developed in the muscles used for descending: quadriceps, adductors and gluteal muscles of one leg and the calf muscle of the other. A large rise in plasma CK occurred in four subjects but no increased Tc-PYP muscle uptake was seen in the quadriceps. In the four subjects with high CK effluxes, increased isotope uptake was seen in the thigh adductors used when stepping down; in the two subjects with the largest CK effluxes there was extensive uptake into the gluteal muscles. Muscle pain preceded and was not well correlated with either the magnitude of the enzyme release or the amount and distribution of increased muscle isotope uptake. We conclude that delayed onset muscle pain, the cause of which remains unknown, is a poor indicator of muscle damage as indicated by circulating muscle enzymes and muscle isotope uptake. Tc-PYP uptake by skeletal muscle can provide useful information about the localisation and time course of muscle damage.     

Comparison of the Effects of Physical Exercise,Cold Acclimation and Repeated Injections of Isoprenaline on Rat Muscle Enzymes

The metabolic effects on rat cardiac and skeletal muscle of a strenuous program of swimming, of cold acclimation and of isoprenaline treatment (0.3 mg/kg daily for 5 five-day weeks) were compared. Exercised and cold-exposed rats gained less body weight than did controls or isoprenaline-treated rats. In all treated groups the heart and the interscapular brown adipose tissue hypertrophied. The size of the adrenals increased only in isoprenaline-treated animals. Cold-acclimation and physical training increased and isoprenaline treatment reduced or did not affect the activities of succinate dehydrogenase, rnalate dehydrogenase and citrate synthase of cardiac muscle. In skeletal muscle all treatments resulted in increased activities of these enzymes. Of the anaerobic enzymes analysed, only the activity of hexokinase increased in response to the treatments used. This increase was the same in cardiac as in skeletal muscle, but it was significantly greater with isoprenaline-treatment than with training or with cold-acclimation. The activities of lactate dehydrogenase and phosphofructokinase did not differ significantly. All treatments improved cold resistance, but only swimming exercise and cold acclimation significantly increased tolerance to exercise. It is concluded that prolonged stimulation of adrenergic β-receptors by catecholamines is responsible for the metabolic changes observed.     

Mitochondrial H2O2 production is reduced with acute and chronic eccentric exercise in rat skeletal muscle

Oxidative stress with acute/chronic exercise has been so far examined using exercise involving a combination of concentric and eccentric contractions, but skeletal muscles are likely to be injured to a greater extent by pliometric contractions. In the present study, the effects of acute and chronic bouts of downhill running exercise on mitochondrial hydrogen peroxide (H2O2) generation (fluorimetric detection of a dimer with homovanillic acid in presence of horseradish peroxidase) and oxygen consumption in conjunction with antioxidant enzymes activity were examined. The results show that acute eccentric exercise was accompanied by a significantly reduced mitochondrial H2O2 production that is likely due to a decrease in complex I of the electron transport chain (ETC). On the other hand, eccentric training leads to positive adaptations, reflected by a higher citrate synthase activity and decreased mitochondrial H2O2 production. The decrease in mitochondrial H2O2 cannot be attributed to alterations in antioxidant capacities but rather to changes in mitochondrial membrane composition characterized by an increased polyunsaturated to saturated fatty acids ratio, and decreased contents in arachidonic acid and plasmalogens. These results suggest that changes in mitochondrial membrane properties with eccentric training can affect H2O2 production by muscle mitochondria. It is hypothesized that these changes resulted in a mild uncoupling sufficient to reduce electron back flow through complex I of the ETC, the major generator of reactive oxygen species by skeletal muscle mitochondria.     

Heterogeneity of response to exercise of rat muscle pyruvate dehydrogenase complex

Muscle glucose uptake is greatly stimulated by moderate exercise, but full oxidation of the glucose to CO₂ depends on the activity of the pyruvate dehydrogenase (PDH) complex. Our aim was to determine how PDH complex in different muscle groups responds to varying periods of moderate exercise. Rats were run on a motor-driven treadmill for 5–30 min and muscle PDH complex activity was determined in heart, diaphragm and red quadriceps muscles after isolation of mitochondria in the presence of inhibitors of PDH complex interconversion. In heart and diaphragm muscle, exercise caused an increase in PDH complex activity after 5 min, but this was followed by a significant decrease in activity as exercise progressed. In red quadriceps muscle, PDH complex activity was reduced after 5 min of exercise and was decreased further as exercise continued. We conclude that increased duration of exercise can lead to reduced PDH complex activity in rat muscles. We propose that this is a consequence of elevated fatty acid oxidation, the products of which stimulate PDH kinase. This implies that increased glycolysis to lactate and increased fatty acid oxidation can simultaneously provide energy for contracting muscle.     

Linking Pulmonary Oxygen Uptake,Muscle Oxygen Utilization and Cellular Metabolism during Exercise

The energy demand imposed by physical exercise on the components of the oxygen transport and utilization system requires a close link between cellular and external respiration in order to maintain ATP homeostasis. Invasive and non-invasive experimental approaches have been used to elucidate mechanisms regulating the balance between oxygen supply and consumption during exercise. Such approaches suggest that the mechanism controlling the various subsystems coupling internal to external respiration are part of a highly redundant and hierarchical multi-scale system. In this work, we present a “systems biology” framework that integrates experimental and theoretical approaches able to provide simultaneously reliable information on the oxygen transport and utilization processes occurring at the various steps in the pathway of oxygen from air to mitochondria, particularly at the onset of exercise. This multi-disciplinary framework provides insights into the relationship between cellular oxygen consumption derived from measurements of muscle oxygenation during exercise and pulmonary oxygen uptake by indirect calorimetry. With a validated model, muscle oxygen dynamic responses is simulated and quantitatively related to cellular metabolism under a variety of conditions.     

Neural control in human muscle fatigue: changes in muscle afferents,motoneurones and motor cortical drive

The metabolic and muscle blood flow response in recovery from exercise is dependent on the type and the duration of the exercise. Immediately after both intense static and dynamical exercise blood flow to the exercised muscles increases suggesting that blood flow is mechanically hindered by muscle contraction. After the initial rise (seconds) muscle blood flow decreases at a moderate rate and the time to reach resting flow levels varies from seconds to more than 30 min. It is unclear as to what causes the elevated blood flow during recovery. A mismatch between the time course of changes in blood flow and oxygen uptake suggests that the blood flow is not directly regulated by the need of oxygen in the exercised muscles. The hyperaemic response may be linked to locally released factors, such as ions and metabolites. However, the signal by which the blood flow is elevated remains unknown. After exercise both pulmonary and muscle oxygen uptake decrease rapidly, but can remain above resting levels for several hours. Resynthesis of substrates such as CP, ATP and glycogen cannot account for the entire excessive post-exercise oxygen uptake (EPOC) in the exercised muscles and the cause of the elevated muscle oxygen uptake in recovery from exercise remains to be assessed.     

Comparison of the effects of physical exercise, cold acclimation and repeated injections of isoprenaline on rat muscle enzymes.

The metabolic effects on rat cardiac and skeletal muscle of a strenous program of swimming, of cold acclimation and of isoprenaline treatment (0.3 mg/kg daily for 5 five-day weeks) were compared. Exercised and cold-exposed rats gained less body weight than did controls or isoprenaline-treated rats. In all treated groups the heart and the intercapular brown adipose tissue hypertrophied. The size of the adrenals increased only in isoprenaline-treated animals. Cold-acclimation and physical training increased and isoprenaline treatment reduced or did not affect the activities of succinate dehydrogenase, malate dehydrogenase and citrate synthase of cardiac muscle. In the skeletal muscle all treatments resulted in increased activities of these enzymes. Of the anaerobic enzymes analysed, only the activity of hexokinase increased in response to the treatements used. This increase was the same in cardiac as in skeletal muscle, but it was significantly greater with isoprenaline-treatment than with training or with cold-acclimation. The activities of lactate dehydrogenase and phosphofructokinase did not differ significantly. All treatments improved cold resistance, but only swimming exercise and cold acclimation significantly increased tolerance to exercise. It is concluded that prolonged stimulation of adrenergic beta-receptors by catecholamines is responsible for the metabolic changes observed.     

Variable resistance versus constant resistance strength training in adult males

Summary Intramuscular triglycerides mobilization during prolonged physical exercise was examined in rats fed ad libitum, in rats fasted for 24 h and in rats treated with nicotinic acid. It has been found that during exercise the intramuscular triglyceride level was markedly reduced only in the red muscle but not in the white and intermediate muscles. Fasting significantly augmented the utilization of triglycerides in the red muscle during exercise. The post-exercise triglyceride level in the red muscle of the rats treated with nicotinic acid was similar to that in the control group whereas blood FFA level, in the nicotinic acid-treated group was much lower than in the control group. Nicotinic acid increased glycogen utilization in the liver and in the skeletal muscles during exercise.It may be concluded that the major cause of the reduction of the triglyceride level in the red muscle during exercise is a developing shortage of available carbohydrates. The greatly elevated blood FFA level during exercise does not seem to have a sparing effect on the intramuscular triglyceride level during exercise. However, it does spare glycogen content in the liver and the skeletal muscles.This work was supported by the Polish Academy of Sciences. project 10.4.2.01.3.2     

Characterization of frog muscle mitochondria.

Studies on oxidative phosphorylation revealed that, in frog skeletal muscle mitochondria (SKMM) from the thigh, the adenosine diphosphate/oxygen ratio (ADP/O) was 2.8 +/- 0.1 SE, and the respiratory control ratio was 9.5 +/- 0.9, with pyruvate/malate as the substrate. Oxygen uptake rate (Qo2) was 225 mumol O2 per minute per gram mitochondrial protein +/- 13; phosphorylation rate (ADP/O X Qo2 X 2) was 1,230 mumol ADP phosphorylation per minute per gram mitochondrial protein +/- 77; and the phosphorylation capacity (phosphorylation rate times tissue mitochondrial protein content) was 3.6 mumol ADP phosphorylated per gram wet weight of muscle +/- 0.2. Tissue mitochondrial protein content was determined by the measurement of nicotinamide adenine dinucleotide (NADH) oxidase activity. Electron microscopy (EM) revealed intact, isolated, energized twisted mitochondria of a condensed form. Frog sartorius muscle mitochondria gave similar oxidative phosphorylation parameters when investigated independently of the rest of the thigh. These values of SKMM respiration from the frog are similar to those values obtained from pigeon and rabbit heart and rat skeletal muscles. However, because of the low NADH-oxidase activity indicating reduced mitochondrial content (this was verified in low-magnification EM pictures), phosphorylation capacity was significantly reduced in frog skeletal muscle mitochondria.